1. Field of the Invention
This invention relates to a combustion knock detection system provided in an internal combustion engine and more particularly to a combustion knock detection system provided in an internal combustion engine incorporated with the so-called traction control system.
2. Description of the Prior Art
In a conventional knock detection system proposed hitherto such as disclosed in a U.S. Pat. No. 5,165,378 by the assignee, a knock sensor is provided at an internal combustion engine to detect a combustion knock which can occur in the engine. In the known system, the vibration of the engine is detected by the sensor at a range of crankshaft angles during which no combustion occurs to determine a reference noise level. Then, the vibration of the engine detected at a second range of crankshaft angles during which combustion occurs so that knock could occur, is compared with the reference noise level. If the vibration detected at the second crank-shaft angle range is found to exceed the reference noise level, it is determined that knock occurs.
More specifically, as illustrated in FIG. 11(a), in crankshaft angles between TDC (top dead center) positions, which are 120 degrees if the engine has six cylinders, the output of the knock sensor becomes great immediately after each TDC position when a combustion occurs as illustrated in FIG. 11(b). Therefore, two gates, named a "noise gate" and a "knock gate", which are corresponding to the aforesaid first and second ranges of crankshaft angles are prepared as shown in (c) of the same figure. The sensor output in the noise gate is averaged to obtain a noise level VNIS and is then multiplied by an amplification gain (factor) AG and a coefficient K which varies with an engine coolant temperature or manifold pressure or the like, to determine the reference noise level KLVL as follows; EQU KLVL=AG.times.K.times.VNIS
Then the sensor output in the knock gate is compared with the reference noise level KLVL in a circuit which produces a pulse each time the sensor output in the knock gate exceeds the reference noise level as illustrated in FIG. 11(d) and (e). The number of pulses is then counted ("3" in the example) and if the counted values have been found to exceed a predetermined reference number, it is determined that a combustion knock occurs.
Apart from the above, recent years have seen increasing traction control systems, referred to as "TCS" hereinafter, in which the engine output torque is forcibly reduced if a driven wheel of the vehicle on which the engine is mounted is found to be slipped or spun, so as to lower the torque given to the wheel and to finally prevent the wheel from slipping or spinning excessively. The engine output torque reduction for the purpose is carried out by adjusting ignition timing in the retard direction, by adjusting an air-fuel ratio in a lean mixture or by discontinuing supply of the fuel to one or more cylinders.
In the TCS, when it operates, the ignition timing retardation or fuel cut and the like will therefore be carried out so that the mechanical vibration of the engine becomes great due to the sudden change in the engine output torque. For that reason, if a combustion knock is to be detected in the manner earlier mentioned, the knock sensor output in the knock gate will become great and there could be a possibility of erroneously detecting a combustion knock. In other words, a combustion knock could be detected although it does not exists actually, which could be a bar for a knock control.